1. Field of the Invention
The present invention relates generally to semiconductor devices, and more particularly, to a semiconductor integrated circuit device employing a polycrystalline silicon thin film as a wiring layer.
2. Description of the Prior Art
A polycrystalline silicon thin film is commonly employed in integrated circuit devices because of the simplicity and convenience of manufacturing such devices, the high degree of integration that can be attained, and the like. One particular application of a polycrystalline silicon thin film to the electrodes and wiring layers of an integrated circuit device is disclosed, for example, in U.S. Pat. No. 3,699,646. However, in integrated circuits in which a polycrystalline silicon thin film is utilized for wiring, there is a shortcoming that the resistance of the thin film is high. A structure having a metal silicide film on a polycrystalline silicon film has been proposed, such as discussed in SOLID-STATE SCIENCE AND TECHNOLOGY, December issue 1975, pp. 1714-1718, but this does not provide a basic solution to the problem.
Namely, the specific resistivity of a polycrystalline silicon wiring layer is hardly reduced to 10.sup.-3 .OMEGA.-cm or lower, even if the impurity concentration therein is increased to a higher possible value, for example, 10.sup.18 atoms/cm.sup.3 or higher. If the thickness of the layer is increased for the purpose of reducing the resistance, the level difference between the upper surface of the polycrystalline layer and the surface of the insulating film on which the polycrystalline layer is formed becomes large, so that the second wiring layer provided on these surfaces is liable to be broken. Therefore, it is not practical to increase the thickness of the polycrystalline silicon layer exceed a certain value. On the other hand, the length of the wiring layer is determined by the device design. Therefore, the wiring layer in the prior art integrated circuit devices inevitably has a high resistance.
Consequently, the design of an integrated circuit device is achieved by means of various compromises such that instead of device arrangement of degree-of-integration predominant type, device arrangement of wiring predominant type is employed at the sacrifice of degree-of-integration, or wirings made of a low-resistance metal, such as aluminum, are modified from a single-layer structure to a double-layer structure by increasing the steps of processing to assure the degree of integration.